A NEW ORAL ISOTOPIC TEST OF CALCIUM ABSORPTION

Evolution of the calcium paradigm: the relation between vitamin D, serum calcium and calcium absorption

Osteoporosis is the index disease for calcium deficiency, just as rickets/osteomalacia is the index disease for vitamin D deficiency, but there is considerable overlap between them. The common explanation for this overlap is that hypovitaminosis D causes malabsorption of calcium which then causes secondary hyperparathyroidism and is effectively the same thing as calcium deficiency. This paradigm is incorrect. Hypovitaminosis D causes secondary hyperparathyroidism at serum calcidiol levels lower than 60 nmol/L long before it causes malabsorption of calcium because serum calcitriol (which controls calcium absorption) is maintained until serum calcidiol falls below 20 nmol/L. This secondary hyperparathyroidism, probably due to loss of a "calcaemic" action of vitamin D on bone first described in 1957, destroys bone and explains why vitamin D insufficiency is a risk factor for osteoporosis. Vitamin D thus plays a central role in the maintenance of the serum (ionised) calcium, which is more important to the organism than the preservation of the skeleton. Bone is sacrificed when absorbed dietary calcium does not match excretion through the skin, kidneys and bowel which is why calcium deficiency causes osteoporosis in experimental animals and, by implication, in humans.

Metabolic and calcium kinetic studies in idiopathic hypercalciuria

Calcium balances and calcium kinetic studies using (47)Ca were performed in nine male patients with idiopathic hypercalciuria and in three normal male subjects. A sharp reduction in calcium intake in eight patients with idiopathic hypercalciuria caused a decrease in urinary calcium excretion, the latter remaining elevated above that reported for normal subjects on a low calcium diet. The hypercalciuric patients had an enlarged miscible calcium pool size, an increased calcium turnover rate, increased bone formation and bone resorption rates, and an elevated true intestinal calcium absorption rate, the increase of the latter three parameters being proportional to the increase of the turnover rate. The fraction of the calcium turnover rate excreted in the urine was elevated whereas that constituted by the endogenous fecal calcium excretion was decreased. Arguments are presented for the concept that the primary abnormality in idiopathic hypercalciuria is neither renal calcium hyperexcretion nor intestinal calcium hyperreabsorption, but a more fundamental disturbance in calcium metabolism of as yet unknown cause, leading to a high calcium turnover.

Intestinal radiocalcium absorption in the goat: measurement by a double-isotope technique

1. Intestinal radiocalcium absorption was measured in goats by a double-isotope technique involving injection of45CaCl2intravenously and47CaCl2into the abomasum. Cumulative absorption of radiocalcium was calculated by deconvolution analysis form curves of plasma radioactivity.

2. Repeated measurements at 2 d intervals gave highly reproducible results (r0.94,P< 0.001). No systematic difference between two consecutive measurements was observed. A good agreement between absorption of radiocalcium from simultaneously administered47CaCl2and45Ca-labelled hay (r0.93,P< 0.001) seems to justify the use of inorganic47Ca as a tracer for Ca in ruminant diets.

3. Two- to three-fold increases in radiocalcium absorption 48 h after oral treatment with 1,25- dihydroxycholecalciferol or leaves ofSolanum malacoxylonshowed the usefulness of the method in situations of rapidly changing Ca absorption.

4. Endogenous adaptations in intestinal radiocalcium absorption from 20 to 43% were observed in lactating goats when Ca intakes decreased from 12 to 4 g/d.

5. It is concluded that the double-isotope technique is a suitable method for studies of Ca absorption in ruminants when tracer is introduced into the abomasum. The test is completed in 3–4 h and may therefore be used in situations where the absorption of Ca undergoes rapid changes.

The bioavailability of dietary calcium

This update focuses on the bioavailability of dietary calcium for humans. Fundamentals of calcium metabolism, intestinal absorption, urinary excretion and balance are recalled. Dietary factors, especially lactose and other milk components, influencing calcium bioavailability at intestinal and renal levels are reviewed. A critical examination of all the methods used for evaluating calcium bioavailability is made. This includes in vitro assays, classical and isotopic balances, urinary excretion, isotope labeling in the urine, plasma and bones, long term evaluation of bone mineralization and the use of biological bone markers. Importance and advantages of animal models are discussed. The state of the art in the comparative bioavailability of calcium in foods is detailed including a comparison of sources of calcium (dairy products and calcium salts) in human studies and in some animal studies, casein phosphopeptides, proteins, lactose and lactase and their relation with calcium bioavailability (in humans and rats). An update on the consumption of dairy products and bone mass is presented. Emphasis on peculiarities and advantages of calcium in milk and dairy products is given.

Calcium balance during pulse alfacalcidol therapy for secondary hyperparathyroidism in CAPD patients treated with 1.0 and 1.25 mmol/L dialysate calcium

Hypercalcemia frequently occurs in continuous ambulatory peritoneal dialysis (CAPD) patients treated with calcium carbonate and vitamin D metabolites. To reduce the incidence of this complication, it has been proposed to use dialysate solutions with a low calcium concentration. However, there is concern that these solutions may lead to a negative calcium balance. We measured calcium balance in 13 CAPD patients with secondary hyperparathyroidism who were treated with calcium carbonate and alfacalcidol, 2 microg twice weekly, while using 1.0- (1.0 group) and 1.25-mmol/L (1.25 group) dialysate calcium solutions. Calcium absorption was measured after the administration of Ca47. Results for the 1.0 (n = 6) and 1.25 (n = 7) groups included fractional calcium absorptions of 0.14 (range, 0.09 to 0.27) and 0.08 (range, 0.03 to 0.40; P = not significant [NS]) and calcium absorptions of 380 +/- 92 and 331 +/- 83 mg/d (P = NS). Dialysate calcium losses were 93 +/- 20 and 91 +/- 26 mg/d, and total calcium losses (dialysate and urine) were 106 +/- 16 and 108 +/- 40 mg/d (P = NS). Calcium balance was positive in all patients (274 +/- 92 and 223 +/- 65 mg/d; P = NS). These data suggest that the use of 1.0- and 1.25-mmol/L calcium solutions in conjunction with calcium carbonate and pulse alfacalcidol therapy is associated with a positive calcium balance in CAPD patients.

Acute effect and mechanism of action of prolactin on the in situ passive calcium absorption in rat

Acute effect of prolactin (PRL) given intraperitoneally 1 h before on calcium fluxes was studied in 6 in situ intestinal segments from weaned, sexually mature and aged rats. In mature rats, PRL increased net passive calcium absorption in jejunum and cecum by 64% and 38%, respectively, by enhancing lumen-to-plasma calcium flux (CaL-P) in the jejunum and by reducing plasma-to-lumen flux (CaP-L) in the cecum. Since PRL-enhanced both CaL-P and CaP-L in the ileum, net calcium absorption did not change. In weaned rats, PRL significantly increased CaL-P in the jejunum but not in the ileum. However, the increase in net absorption was not significant. In contrast, effect of PRL was not seen in aged rats. By demonstrating an absence of PRL action on jejunal calcium fluxes when sodium-free test solution was used, we reconfirmed the sodium-dependent PRL action on passive calcium absorption. The PRL-induced parallel increase in the lumen-to-plasma transport of 45Ca and [3H]mannitol indicated that PRL action was to increase the paracellular transport of calcium.

Plasma kinetics of magnesium and calcium stable isotope tracers in a human subject after simultaneous oral ingestion of 25Mg and 44Ca determined by thermal ionization mass spectrometry

The plasma concentrations of stable isotope tracers of magnesium (25Mg) and calcium (44Ca) were determined during a 50 h period, after simultaneous oral tracer ingestion in an adult man. The measurements were performed by magnetic sector field mass spectrometry using surface ionization ion sources. The data were used to evaluate the potential of this method in long-term kinetic studies. It is concluded that under the experimental conditions specified, the isotopic tracers will be detectable in blood plasma with sufficient precision and accuracy for approximately 600 h after oral ingestion. The differences in the plasma concentration curves of 25Mg and 44Ca were analysed by means of the convolution integral method. These differences indicate inhomogeneities in the distribution of absorption activity relative to magnesium and calcium within the upper small intestine.

A calcium model with random absorption: a stochastic approach

Absorption of calcium, or any mineral, by the body is subject to the random fluctuations typical of diffusion through membranes. In this paper we consider the absorption of calcium from the gut as a white noise process added to the deterministic model of Sen & Mohr (1990, J. theor. Biol. 142, 179-188). The first two moments for the amount of calcium in the extracellular fluid (ECF) have been derived using the Ito Calculus. A confidence interval for the total amount of calcium in the ECF is constructed. The equations for the first two moments of the fraction of dose calcium in the ECF are also given. Suggestions are made for the collection of experimental data in a form which should be helpful in investigating the magnitude of the stochastic effect.

Further studies on acute effect of prolactin on intestinal calcium absorption in rat

The present investigation aimed to (1) reconfirm the acute effect of prolactin on intestinal calcium absorption in the rat, (2) evaluate the influence of age on prolactin action, and (3) further investigate the mechanisms of prolactin action on calcium absorption. The intestinal calcium absorption in mature rats was evaluated by measuring the plasma 45Ca content at intervals during a 1-h period following intragastric administration of a 45Ca-containing test solution and by measuring tibial 45Ca content in weaned rats. By using weaned (50-70 g), sexually mature (180-200g) or aged (greater than 250 g) female Wistar rats, we demonstrated that calcium absorption was inversely related to age. Prolactin at a dose of 0.02 mg/100 g body weight, administered intraperitoneally 1 h before Ca administration, significantly enhanced calcium absorption in sexually mature and weaned rats but not in aged rats. At a luminal calcium concentration of 5 mM, prolactin had no effect when the luminal solution was either CaCl2 alone or an Na-free electrolyte solution. On the other hand, prolactin significantly increased the plasma 45Ca content by 57, 42 and 28% at 5, 15 and 30 min respectively, when the electrolyte solution contained sodium. In contrast, in the presence of 0.5 mM calcium, the enhancing effect of prolactin on calcium absorption was not dependent on the presence of sodium. Compared with the saline control, the plasma 45Ca content in prolactin treated animals was significantly elevated by 63% at 5 min when the test solution was a sodium-free electrolyte solution.

A kinetic model for calcium distribution

Models for the distribution of minerals in the body are of interest as they allow researchers to trace the effect of a dose on mineral levels in plasma, storage and other compartments. Limited models are available in the literature for tracing the distribution of a calcium dose through a short time period. We propose a more general kinetic model which includes both limited absorption through the gut and loss of calcium via excretion. This new method has the advantages of giving reasonable results over moderate time periods, and allowing the extrapolation of calcium levels in extracellular fluid and storage. We fit the model to published data in order to obtain typical parameter values. These values are then used to analyze the implications of the model regarding the effect of calcium dose on calcium levels in various compartments.

Simple test of intestinal calcium absorption measured by stable strontium

A clinical test of intestinal calcium absorption has been developed using non-radioactive stable strontium as a calcium tracer. In nine elderly subjects there was a close correlation between the fractional absorption of strontium and radioactive calcium (45Ca) during a five hour period after the simultaneous oral administration of the two tracers. Comparable precision was achieved with each tracer in six subjects in whom the test was repeated after two weeks. The effect of food on strontium absorption was examined in a further 33 normal subjects (age 21-60 years), and the administration of the strontium with a standard breakfast was shown to reduce the variance at individual time points. A simplified test in which serum strontium concentration was measured four hours after the oral dose given with a standard breakfast was adopted as the routine procedure. The normal range (mean (2 SD], established over 97 tests in 53 patients, was 7.0-18.0% of the dose in the extracellular fluid. A further 30 patients with possible disorders of calcium absorption (10 with primary hyperparathyroidism and 20 with coeliac disease) were studied by this standard test. In both groups of patients the mean four hour strontium values were significantly different from normal. This standard strontium absorption test allows assessment of calcium absorption with sufficient sensitivity and precision to have a wide application in clinical practice.

Effect of zinc supplements on the intestinal absorption of calcium

Pharmacologic doses of zinc are widely used as zinc supplements. As calcium and zinc may compete for common absorption sites, a study was carried out on the effect of a pharmacologic dose of zinc on the intestinal absorption of calcium in adult males. The analyzed dietary zinc intake in the control studies was normal, averaging 14.6 mg/day. During the high zinc study, 140 mg zinc as the sulfate was added daily for time periods ranging from 17 to 71 days. The studies were carried out during both a low calcium intake averaging 230 mg/day and during a normal calcium intake of 800 mg/day. Calcium absorption studies were carried out during the normal and high zinc intake by using an oral tracer dose of Ca47 and determining plasma levels and urinary and fecal excretions of Ca47. The study has shown that, during zinc supplementation, the intestinal absorption of calcium was significantly lower during a low calcium intake than in the control study, 39.3% vs 61% respectively, p less than 0.001. However, during a normal calcium intake of 800 mg/day, the high zinc intake had no significant effect on the intestinal absorption of calcium. These studies have shown that the high zinc intake decreased the intestinal absorption of calcium during a low calcium intake but not during a normal calcium intake.

25-Hydroxycholecalciferol in experimental osteoporosis

The authors, using the experimental pattern of bone rarefaction induced by a low calcium diet, tried to determine if 25-hydroxycholecalciferol, administered at doses and according to procedures similar to those used in osteoporosis, might interfere in such a process of rarefaction. The statistical evaluation of the results concerning the size of the amount of spongy bone presented evidence for the sharp decrease in the specific bone volume in rats placed on a low calcium diet as compared to control rats. Such a difference is found even between rats treated with 25-OH-D3. Nonetheless, the authors reported a difference--although not statistically significant--that indicated an increase in the amount of bone in rats on a normal diet but treated with vitamin D as compared to control rats.

IN CONCLUSION

25-OH-D3 by itself, the diet being the same, was not able to ameliorate the amount of bone.

A theoretical model for calcium absorption from the intestinal lumen

A theoretical model is developed for the absorption of calcium in the human gut and a mathematical description of the model is written. For simplicity, only three compartments are considered to derive a kinetic equation which can be fit to plasma activity measurements following oral ingestion of radiotracer calcium. Initial tests show good success in correlating total fractional absorption of calcium.

Calcium and vitamin D metabolism in Guamanian Chamorros with amyotrophic lateral sclerosis and parkinsonism-dementia

We evaluated 16 Guamanian Chamorros with amyotrophic lateral sclerosis and 33 patients with parkinsonism-dementia for disturbances of calcium and vitamin D metabolism. The serum immunoreactive parathyroid hormone level was mildly elevated in 6 patients with amyotrophic lateral sclerosis and in 5 patients with parkinsonism-dementia. There were significant positive correlations between serum immunoreactive parathyroid levels and duration of illness in male patients with motor neuron disease, but not in female patients or in patients with parkinsonism-dementia. Intestinal absorption of calcium, as assessed by serum and urinary activity of calcium 47 following oral administration, was decreased in 2 patients with amyotrophic lateral sclerosis and in 4 patients with parkinsonism-dementia, all of whom had low levels of serum 1,25-dihydroxyvitamin D. Reductions in cortical bone mass were striking in patients with motor neuron disease. A significant negative correlation was found between the percentage of cortical area of the second metacarpal bone and muscle atrophy and weakness, and significant positive correlations were found between degree of immobility and ratio of urinary hydroxyproline to creatinine in patients with amyotrophic lateral sclerosis and parkinsonism-dementia. In general, abnormalities in calcium metabolism were subtle. Thus, if the demonstrated deposition of metals, particularly calcium and aluminum, in central nervous system tissues of Guamanians with these two conditions is a cause of the diseases and of the early appearance of neurofibrillary tangles in neurons, the accumulation has apparently occurred long before onset of symptoms, and detectable abnormalities of calcium and vitamin D metabolism may already have been corrected.

Parathyroid hormone and calcium metabolism in generalized scleroderma. Increased PTH level and secondary hyperparathyroidism in patients with aberrant calcifications. Prophylactic treatment of calcinosis

Parathyroid hormone (PTH) in serum and biochemical parameters of calcium metabolism were analysed in 45 patients with systemic sclerosis. Calcification of the skin and subcutaneous tissue was assessed by X-ray examination of the hands. Analyses disclosed secondary hyperparathyroidism (increased PTH in serum, low calcium 'ion' in serum, decreased urinary excretion of calcium and phosphate), in particular in patients with calcinosis (P less than 0.05) as compared to those with no calcinosis. The duration of systemic sclerosis was longer in patients with calcinosis (P less than 0.05). The calcinosis type of systemic sclerosis is characterized by secondary hyperparathyroidism developed during the progression of the disease. A hypothesis is made regarding calcium metabolism in the early no-calcinosis (with increased synthesis of Vitamin D) and late calcinosis types. PTH may stimulate aberrant calcification. The hypothesis implicates that prophylactic treatment with Vitamin D in low dose may prevent calcinosis.

Controlled trial of 1apha-hydroxycholecalciferol in chronic renal failure

24 patients with chronic renal failure (glomerular filtration-rate (G.F.R.) 5-25 ml/min) participated in a double-blind placebo-controlled trial of the effects of 1 alpha-hydroxycholecalciferol (1alpha-H.C.C.) 1 mug daily for eleven weeks. This treatment induced significant increases in the intestinal absorption of calcium and in plasma-calcium which reached normal levels within two weeks. It also induced a significant reduction of the raised serum levels of parathyroid hormone. No significant changes were induced in plasma-phosphorus, plasma-alkaline-phosphatase, or in the degree of bone mineralisation as measured by the phosphorus/hydroxyproline ratio in bone. The bone mineral content in the forearm measured by photon absorptiometry decreased to the same extent in the 1alpha-H.C.C. groups and in the placebo group. The fall in G.F.R. over eleven weeks was 2-5 times greater in the 1alpha-H.C.C. group than in the placebo group, but this difference was not significant. It is concluded that 1alpha-H.C.C. treatment in chronic renal failure does not affect the progressive loss of calcium from bone despite normalisation of plasma-calcium.

Calculation of three hours calcium absorption from a double isotope test; a simplified method

For the rapid evaluation of calcium absorption in human subjects several isotopic tests have been proposed. Among them, the double isotope test based on the concurrent oral and intravenous administration of calcium isotopes seems the most reliable. However, the computations involved in this test, a deconvolution of the two specific-activity functions obtained, are quite extensive. By simulating these functions we developed a simple formula to calculate the absorption in the first three hours of the test. Applied to the data of 79 patients the result differed less than the experimental error from the absorption computed with the more elaborate deconvolution method.

The acute effect of sodium cellulose phosphate on intestinal absorption and urinary excretion of calcium in man

Intestinal 47Ca absorption has been determined from blood and stool radioactivity after oral administration of the isotope in nine patients before and during cellulose phosphate treatment. Oral administration of 5 g cellulose phosphate concomitant with 47Ca in 100 mg "carrier" calcium as CaCl2 decreased 47Ca absorption by 80 per cent. Cellulose phosphate, 5 g three times daily, decreased urinary excretion of non-radioactive calcium by 47 percent. Urinary magnesium excretion decreased by 47 percent whereas urinary phosphorus excretion increased by 67%. Calcium and magnesium excretion in urine decrease because cellulose phosphate binds divalent cations within the GI tract. The increased phosphorus excretion is probably due to partial hydrolysis of the substance in the gut.

Effects of therapy on rate of absorption of calcium from gut in disorders of calcium homoeostasis

A development of the double-tracer calcium absorption test, in which a function is derived for single-passage entry rate from the gut with time, has been assessed in various disorders of calcium homoeostasis. Single tests are diagnostically useful in calcium malabsorption but because of the wide spectrum of normal values a single test can only moderately enhance the accuracy of the diagnosis of hypercalcaemic states made primarily by other methods. On the other hand serial studies in the same subject can be much more informative because the test is normally highly reproducible if conditions are unchanged. Serial studies play an important part in assessing the efficacy of established methods of treatment in disorders of calcium homoeostasis and should be particularly helpful in studying the effects of new forms of therapy on the absorption process.

Treatment of calcinosis circumscripta with probenecid

A 23-year-old woman presented with an 11-year history of peripheral joint pains and stiffness slowly progressing to the development of gross calcinosis circumscripta. She had no definite evidence of systemic scleroderma. She was first treated for one year with frequent intravenous sodium EDTA, which increased urinary calcium and enhanced negative calcium balance but did not lessen her calcinosis. She was then treated with probenecid up to 2 g daily and her calcinosis lessened steadily over three years. Plasma levels of both phosphorus and urate fell during this time.

Intestinal absorption of calcium-47 after treatment with oral oestrogen-gestogens in senile osteoporosis

Intestinal absorption of radiocalcium was measured in 15 postmenopausal women with osteoporosis before and after six months' treatment with an oral oestrogen-gestogen combination. Comparison with a control group indicated a significant improvement in intestinal absorption after treatment. Though there is no evidence that oestrogens have an anabolic effect on human bone, these results indicate that they affect the intestinal absorption of calcium directly.

Renal involvement in sarcoidosis

Among 152 sarcoidosis patients, 11% had hypercalcaemia, 62% had hypercalciuria and 13·8% had at least one renal stone. Impairment of renal function was mostly conspicuous in patients with hypercalcaemia. Twenty-five successful percutaneous renal biopsies were performed. Epithelioid granulomas were seen in ten cases, with inflammatory reaction more prominent than in other organs involved by sarcoidosis, due to simultaneous pyelonephritis. Thickening and hyalinization of capsular membrane and glomeruli as well as arterial walls were frequently encountered. Amyloid deposits could be seen in one case with chronic lung infection. Histopathological changes specific of sarcoidosis are related to an abnormal amount of serum gammaglobulins and calciuria exceeding 200 mg in 24 hr, which seem to be the best tests of involvement by the disease.

Tubular reabsorption of calcium was low in two patients with hypercalciuria, which was corrected after corticosteroid treatment.

High intestinal calcium absorption was lowered after hydrochlorothiazide had decreased urine calcium.

Study of calcium absorption in man: a kinetic analysis and physiologic model

A physical model of calcium absorption was developed from analysis of data obtained on 23 subjects, including 13 patients having a variety of abnormalities of calcium metabolism. The model was tested and found consistent in all subjects studied. This technique provides a quantitative description of the rate of entry of oral dose of (47)Ca into the circulation as a function of time by analysis of serum or forearm radioactivity in response to intravenous and oral administration of (47)Ca. The kinetics of the absorption process as proposed by the model are characterized by an initial delay phase of 15-20 min, by a maximal rate of absorption at 40-60 min after ingestion, and by 95% completion of the absorption within 2(1/2) hr. Partial identification of the physiological counterparts of the model was possible by introduction of the isotope at various levels of the gut. Although the region of the duodenum was found to have the greatest rate of absorption per unit length in normal subjects, it was least responsive to stimulation by parathyroid hormone and suppression by calcium loading. Furthermore, the response of the gut to parathyroid hormone was delayed, whereas the suppression of absorption by intravenous or oral calcium loading was rapid and dramatic. The implications of these observations are discussed.

Absorption of calcium measured by intubation and perfusion of the intact hyman small intestine

Absorption of calcium was measured by direct intubation and perfusion of the small intestine in 10 volunteer normal adult subjects, two adults with celiac-sprue, and one with a parathyroid adenoma. A total of 60 studies were completed using one of two different levels, duodenojejunum or ileum. Solutions containing stable calcium, radiocalcium(47), and a nonabsorbable dilution-concentration marker, polyethylene glycol, were infused at a uniform rate via the proximal lumen of a triple-lumen polyvinyl tube. The mixed intraluminal contents were continuously sampled by siphonage from two distal sites, 10 and 60 cm below the point of infusion. Unidirectional flux rates, lumen to blood and blood to lumen, and net absorption of calcium for the 50 cm segment of small intestine between the two collection sites were calculated from the measured changes in concentration of stable calcium, calcium-47, and polyethylene glycol.Flux of calcium from lumen to blood in the duodenojejunum of normal subjects was appreciable even when the concentration of calcium in the perfusate was below that of extracellular fluid and, as the intraluminal concentration of calcium was increased through a range of 0.5-3.5 mumoles/ml, was positively correlated, ranging from 1.9 to 7.0 mumoles/min per 50 cm. Repeated studies of individual subjects demonstrated a consistent pattern of absorptive efficiency in each, but significant variability from person to person. Flux from lumen to blood in the ileal segment occurred at a much lower rate than that found in the proximal intestine, and there was not a significant dependence upon intraluminal calcium concentration. The opposite flux, from blood to lumen, was low both in the duodenojejunum and ileum (average 0.76 mumoles/min per 50 cm) and was independent of the intraluminal calcium concentration. Unidirectional flux, lumen to blood, from the duodenojejunum was not altered by parathyroid extract administered at the time of the infusion, but was accelerated in the subject with a parathyroid adenoma and markedly reduced in the two subjects with celiac-sprue.